Yearly Archives: 2014

Discrete wave operator for causets

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Joseph Samuel

Joseph Samuel is an Editorial Board member of Classical and Quantum Gravity and a Professor at the Raman Research Institute, Bangalore, India

This paper leads to a discrete action functional on causets.

Lisa Glaser presents some tidy results in the definition of the discrete d’Alembertian operator on a causet in any dimension.

The causet approach to quantum gravity was pioneered by Rafael Sorkin in the 1980s. It approximates the space-time continuum by a discrete structure–a set with a partial order. The causet approach is marked by its minimalist philosophy, capturing Lorentzian manifolds in a discrete net with just Continue reading →

Interview with Dr Anna Heffernan: winner of the 2014 IOP Gravitational Physics Group’s thesis prize, co-sponsored by CQG

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Anna Heffernan

Dr Anna Heffernan, Advanced Concepts Team, European Space Agency

What led you into science and your chosen area of research?

I’ve always enjoyed Mathematics and learning about the world around me. General relativity allows us to use mathematics to probe the very space time in which we exist – I find this fascinating, and it means I also get to play with equations for a living.

What do you find most interesting about this subject?

The fundamental questions it both addresses and raises. I also like the idea that with the emergence of gravitational wave astronomy, general relativity will soon be used as a tool to explore our universe.

Can you tell us a little bit about the work in your thesis?

Sure, my thesis was on the self-force problem – this is when Continue reading →

IOP GPG thesis prize awarded to Dr Anna Heffernan

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Clifford Will

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity, Distinguished Professor of Physics at the University of Florida, Chercheur Associé at the Institut d’Astrophysique de Paris, and James McDonnell Professor of Space Sciences Emeritus at Washington University in St. Louis.

I am delighted to announce that the IOP’s Gravitational Physics Group’s thesis prize for 2014, co-sponsored by Classical and Quantum Gravity, has been awarded to Dr Anna Heffernan for her eloquently written thesis on the self-force problem in gravitational physics, and for her detailed calculation of the singular component of the divergent fields that arise.

Dr Heffernan obtained her PhD from University College Dublin, under the supervision of Prof. Adrian Ottewill.  She currently works at Continue reading →

Continuous symmetries in discrete space?

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Martin Bojowald

Martin Bojowald is a Professor of Physics at Pennsylvania State University, PA,
USA

Violating Lorentz symmetries can be dangerous. A new model assesses this threat in loop quantum gravity.

Discrete space is attractive in quantization attempts of gravity, but it implies the great danger of violating Lorentz transformations. Any approach must show that modifications of symmetries by discrete space are tame enough for predictions to be consistent with continuum low-energy physics. In the CQG paper, Sandipan Sengupta constructs an encouraging model using Continue reading →

Asymptotic flatness and quantum geometry

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Sandipan Sengupta

Sandipan Sengupta is a post-doctoral fellow at the Raman Research Institute in Bangalore, India

From the perspective of quantum gravity, the spacetime is smooth only in an effective sense, and is expected to exhibit a discrete structure at suitably small length scales. Within the gauge theoretic formulation of gravity, there are certain kinematical states which provide an elegant realization of such a scenario. These are known as the spin-network states, and are used extensively in certain quantization approaches, e.g. Loop Quantum Gravity (LQG). However, since these states correspond to a spatially discrete quantum geometry, they cannnot be used to capture the notion of a classical spacetime continuum. This leads to a serious obstacle towards a quantization of Continue reading →

Homogeneous cosmological model from a discrete matter distribution

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Mikolaj Korzynski

Mikolaj Korzynski is an Assistant Professor at the Center of Theoretical Physics
of the Polish Academy of Sciences, Warsaw

How does a homogeneous FLRW metric arise from a cosmological model with black holes as the only source of gravitational field?

In astrophysical applications of general relativity we often need to apply the Einstein’s field equations to situations where the matter distribution, and consequently also the metric tensor, has a complicated form with relatively smooth large scale behavior and a  complicated structure on smaller scales. The problems of this kind are usually approached in the following way: instead of solving the equations directly we apply them to an idealized metric with the small-scale structure removed by Continue reading →

Melvin magnetic cosmologies

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Magnetic fields are ubiquitous in the universe – observed on scales ranging from stellar, through galactic and beyond – and are key to the physics of dramatic astrophysical objects such as pulsars and active galactic nuclei. Meanwhile, the origin of large-scale magnetic fields is still a topic of great debate in the cosmological literature.

Our recent CQG article presents a new family of exact solutions to the Einstein-Maxwell equations for cosmological magnetic fields. These solutions are both inhomogeneous and anisotropic, with the magnetic field having nontrivial dependence on Continue reading →

Probing the notion of gravitational entropy in inhomogeneous cosmologies

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Roberto Sussman

Dr Roberto A Sussman is a senior researcher in Theoretical Cosmology at the Institute for Nuclear Sciences (ICN) of the National University of Mexico (UNAM).

One of the long standing open problems in General Relativity is to find a self-consistent theoretically robust definition of a classical “gravitational” entropy, which is distinct (though possibly connected) to the entropy of the field sources (hydrodynamical or non-collisional) and to holographic and black hole entropies. Current research has produced two main classical gravitational entropy proposals: one by Clifton, Ellis and Tavakol, based on an effective construction from the “free” gravitational field associated to the Bell-Robinson tensor (the CET proposal), the other, by Hosoya and Buchert, is based on the Kullback-Leibler functional of Information Theory (the HB proposal).

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High-order fully general-relativistic hydrodynamics: new approaches and tests

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Pablo Laguna

Pablo Laguna is the Chair of the School of Physics at Georgia Tech

As we approach the era of gravitational-wave astrophysics driven by observations, it is imperative to have general-relativistic hydrodynamic codes capable of revealing in exquisite detail phenomena driven by strong dynamical gravity.

In this paper, Radice, Rezzolla and Galeazzi introduce a new approach to build a code, called WhiskyTHC, with the potential to help deliver that. The new approach borrows elements from the Whisky and Template Hydrodynamics codes. The Whisky code is widely used by the numerical relativity community, and the Continue reading →

First occurrence of a double layer in a gravity theory found

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José Senovilla

Jose M. M. Senovilla is a full Professor of Theoretical Physics at the University of the Basque Country (UPV/EHU) in Bilbao, Spain

Gravitational double layers turn out to be feasible in quadratic theories of gravity. New physics arises.

Double layers (DL) may be found in several disciplines: in biology separating two different forms of matter, in chemistry as interfaces between different phases (liquid and solid), or in physics when two laminar parallel shells with opposite electric charges are found next to each other. DL are especially important in plasma and cellular physics, representing abrupt drops in the electric potential by which the cell, or plasma, “protects” itself from the environment.

However, gravitational DL were nowhere to be found in gravitational physics, until now. Continue reading →